Two collimated laser beams, wavelength O-514 μm are overlapped within a crystal of heavily iron doped lithium niobate to form a one-dimensional volume holographic grating, a few mm long, designed to behave as a highly selective filter at near infra-red wavelengths. A novel recording geometry is described and a variety of diagnostic experiments are undertaken to determine the main characteristics of the grating. A filter fabricated using this method is used to obtain single mode operation from a 1-55 μm semiconductor laser. The historical survey of holography contained in chapter one emphasizes the dynamic properties and applications of photorefractive recording materials. Standard results of two mathematical theories, kinematic and coupled-wave, are used in chapter two to predict the properties of a Bragg grating when replayed with infra-red light. Chapter three details the recording materials suitable for an infra-red filter, followed by a discussion on the concept of 'scaling'. The recording mechanism of iron doped lithium niobate, including its fixing and erasure process, is presented in chapter four as a basis for selecting the most advantageous crystal characteristics. Mathematical models describing the complex dynamic recording process in iron doped lithium niobate are introduced in chapter five. A novel recording geometry involving the use of two prisms is critically described in chapter six, from initial conception to final implementation. The performance of the grating as an infra-red filter is assessed in chapter seven by measuring the angular response of the transmitted and diffracted beams at a wavelength of 1-152 microns. Chapter eight discusses the applications for volume holographic filters, with particular reference to one example. In chapter nine general conclusions are drawn and future directions for research are suggested. A number of appendices are also included.

<p> In this work, tunable Mid-Infrared (IR) light sources based on quasi-phase matched (QPM) difference frequency generation (DFG) by periodically poled lithium niobate (PPLN) crystals are studied. The theory of DFG and the characteristics of lithium niobate crystals are described and analyzed. Characteristics of the wavelength tuning of QPM DFG by PPLN crystals are studied. In order to analyze in detail, simulation and experimental data of the widely tunable mid-IR laser source around 2 1- μm to 5 1- μm wavelength are presented. The simulations of DFG process by PPLN are conducted based on the nonlinear optics reported. In the experiment, a 1.064 μm Nd:YAG laser and a tunable Ti:sapphire laser are employed as the signal and pump lasers, respectively. Based on the studies of the wavelength tuning characteristics at different temperatures, an optimization procedure to achieve a maximum wavelength tuning range is proposed. The potential applications in gas detection of the mid-IR source are also described briefly. Recommendation for future works and potential applications of the PPLN DFG based mid-IR lasers are discussed. </p> / Thesis / Master of Applied Science (MASc)

Two-port polarization independent electro-optically wavelength tunable filters based on asymmetric Mach-Zehnder structure in LiNbO3 substrate have been developed for 1.55μm application. The operation principle is based on Mach-Zehnder interference and TE↔TM mode conversion. Fabrication parameters for channel waveguides, interferometers and mode converters have been optimized. 7μm wide single mode straight channel waveguides were produced by diffusing 1050-1100A thick Ti into LiNbO3 substrate. Insertion loss of 3.6dB was achieved for both TE and TM polarization. Mach-Zehnder interferometer performance was optimized by testing the Y-branch on samples cut in half length compared to complete device. Best results were obtained from samples that were produced by diffusion at 1025 degC for 11 hours of 1050A thick Ti film, and by diffusion at 1025 degC for 12 hours of 1090A thick Ti film. Metal electrodes were added to one arm of asymmetric Mach-Zehnder interferometers to evaluate electro-optic modulation. Modulation depth as high as 99.6% for TE mode and 98.9% for TM mode were obtained. TE↔TM mode conversion was demonstrated on straight channel waveguides with conversion efficiency greater than 96% utilizing 500 strain inducing SiO2 grating pads with 21μm spatial period. Two-port polarization independent electro-optically tunable wavelength filters were produced based on the optimized parameters described above. The -3dB bandwidth of the filter is 2.4nm. The nearest side lobe to the main peak is more than 13dB below the central lobe for both TE polarization and TM polarization. A thermal tuning rate of -0.765nm/degC is obtained. An electrical tuning range of 12.8nm and a tuning rate of 0.08nm/V were achieved.

A tunable guided-wave optical filter that performs spectral slicing at the 1530nm wavelength regime in Ti:LiNbO3 was proposed and fabricated. It is aimed at minimizing crosstalk between channels in dense wavelength division multiplexing (DWDM) optical network applications. The design utilizes a sparse grating allowing the selection of equally spaced channels in the frequency domain. Between selected channels, equally spaced nulls are also produced. The sparse grating is formed by using N coupling regions with different lengths along the direction of propagation of light in the waveguide, generating N-1 equally spaced nulls between adjacent selected channels. The distance between the centers of adjacent coupling regions is kept constant. The filtering is based on codirectional polarization coupling between transverse electric (TE) and transverse magnetic (TM) orthogonal modes in a waveguide through an overlay of strain-induced index grating, via the strain-optic effect. Two types of devices were fabricated. In the first type, the sparse gratings were produced on straight channel waveguides. Selected channels emerge from the device in a polarization state orthogonal to the input and a polarizer is needed to observe the filtered light. For the second type, an asymmetric Mach-Zehnder interferometer configuration was used to eliminate the need of the polarizer at the output, and yields an output response that is polarization independent. Both types of devices were fabricated on x-cut y-propagating LiNbO3 substrates, with N = 6 strain-induced coupling regions. The single mode channel waveguides were formed by Ti diffusion. Electrode patterns centered about the optical waveguide were defined by liftoff. In the straight channel devices, insertion loss was less than 2.5 dB on a 43 mm sample. The 3-dB channel bandwidth of the selected channels is approximately 1.0 nm. Devices were tuned thermally as well as by voltage application to surface electrodes resulting in tuning rates of 1.0 nm/oC and 0.04148 nm/V, respectively. In the polarization independent device the insertion loss for the phase-matched wavelength was 5.3 dB on a 53 mm long chip. The 3-dB bandwidth was also ~1.0 nm and the thermal tuning rate 1.0 nm/oC. The experimental results are in good agreement with design theory.

There is a drive for improving the surface uniformity of optical waveguide devices in the photonics lab. This report focuses on the exploration of annealed proton exchange (APE) waveguide fabrication on lithium niobate crystal as a method of producing optical waveguides. These waveguides aim to have little variation in step height or surface roughness in the transition area from the waveguide location to that of the bulk crystal, providing a uniform surface amenable to vertical device integration. This is a substantial improvement over the titanium diffused waveguide process, which can have surface variations in excess of 100nm. It is anticipated that the smoother surface will enable light to couple more easily into photonic devices, such as ring resonators, as compared to the current Ti diffused waveguide process. This work explores the design and fabrication aspects of annealed proton exchange waveguides. A review of literature on modeling hydrogen diffusion into lithium niobate is presented, as well as computer models for simulating the bidimensional fractional hydrogen proton concentration distribution. This is used to determine the change in refractive index of the waveguide needed to simulate the mode propagation and profile in the device. Fabrication processes involved in proton exchange waveguide formation are outlined, and measurements for working devices are presented. Best case loss for current devices are 0.5 dB/cm. These samples exhibit smooth surfaces with only ±60A in variation of surface uniformity. Concluding remarks present ideas to further the work by lowering propagation losses, improving mode matching to single mode fiber, and improving the consistency of fabrication conditions.

Two-port polarization independent electro-optically wavelength tunable filters based on asymmetric Mach-Zehnder structure in LiNbO3 substrate have been developed for 1.55μm application. The operation principle is based on Mach-Zehnder interference and TE↔TM mode conversion. Fabrication parameters for channel waveguides, interferometers and mode converters have been optimized. 7μm wide single mode straight channel waveguides were produced by diffusing 1050-1100A thick Ti into LiNbO3 substrate. Insertion loss of 3.6dB was achieved for both TE and TM polarization. Mach-Zehnder interferometer performance was optimized by testing the Y-branch on samples cut in half length compared to complete device. Best results were obtained from samples that were produced by diffusion at 1025 degC for 11 hours of 1050A thick Ti film, and by diffusion at 1025 degC for 12 hours of 1090A thick Ti film. Metal electrodes were added to one arm of asymmetric Mach-Zehnder interferometers to evaluate electro-optic modulation. Modulation depth as high as 99.6% for TE mode and 98.9% for TM mode were obtained. TE↔TM mode conversion was demonstrated on straight channel waveguides with conversion efficiency greater than 96% utilizing 500 strain inducing SiO2 grating pads with 21μm spatial period. Two-port polarization independent electro-optically tunable wavelength filters were produced based on the optimized parameters described above. The -3dB bandwidth of the filter is 2.4nm. The nearest side lobe to the main peak is more than 13dB below the central lobe for both TE polarization and TM polarization. A thermal tuning rate of -0.765nm/degC is obtained. An electrical tuning range of 12.8nm and a tuning rate of 0.08nm/V were achieved.